The object of this research is to study the histochemistry of cadmium and zinc at the ultrastructural level: 1) to prove the nature and usefulness of cadmium iodide osmium (CdIOs) as a research tool for the investigation of the cellular transport of calcium in gut and kidney, the localization of calcium in the cellular initiation of mineralization in bone and cartilage, and to improve localization of some of the enzymes involved in these processes by using cadmium chloride or iodide as the trapping agent in lieu of cobalt or strontium; 2) to prove the nature and usefulness of zinc iodide osmium (ZnIOs) as a label for the progressive steps in the pathway of differentiation in bone cell lines (osteoblast-osteocyte vs. osteoclast) and to investigate its use as an indicator of bone t-Rna and protein synthesis; 3) to apply these findings to bone remodeling cyclic longitudinal growth and repair. In the cadmium iodide osmium studies, variation of pH and presoaking in cadmium chloride or cadmium iodide will be used to find the maximum precipitation possible. Rat or embryonic and neonatal chick studies with cadmium-osmium will be used to show the accumulation of phospholipids in the cytoplasm of the gut cell. Bacteria which are known either to contain a high concentration of phosphatidyl serine, a calcium binding and suggested transporting lipid, or to mineralize, will test whether cadmium iodide osmium has any stain specificity for particular phospholipids. Cadmium will be used as a "trapping agent" in Ca ion-ATPase and alkaline phosphatase. In the zinc iodide osmium studies, sulfur transferase activity will be used as an enzyme to measure t-RNA (protein synthesis). S35-mercaptopyruvate and H3-proline will be used in E.M. autoradiography of osteoblasts to correlate radioactivity with staining density. These combined findings will be applied to callus repair and the associated regional accelerating phenomena which occur in adjacent cortical bone in the fractured dog rib and to cyclic bone growth in the young adult female rat.